The invention involves an arrangement of the fractionation trays used in distillation columns which separate volatile chemical compounds via fractional distillation. More particularly, the invention involves fractionation trays having downcomers oriented at a 45 degree angle to either a support beam that spans the diameter of a distillation column or to a divided wall in the case where the column is a divided wall column.
Fractionation trays are widely used in the petrochemical and petroleum refining industries to promote the multistage vapor-liquid contacting performed in fractionation columns. The normal configuration of a fractionation column includes about 10 to 120 individual trays. Normally the structure of each tray in the column is the same. The trays are mounted horizontally at uniform vertical distances referred to as the tray spacing of the column. This distance may vary within different parts of the column but is normally considered constant. The trays are normally supported by a ring welded or bolted to the inner surface of the column.
Fractionation trays previous to the present invention have a unit cell design due to a 90 degree rotation between trays which is ideal for using within a divided wall column. However, the wall makes it impossible to have the same hydraulic tray design for odd and even trays if the wall is parallel or perpendicular to the downcomer orientation. This is typical orientation that people would expect. The differences between odd and even trays become even greater if the dividing wall is slightly off of center. Therefore, the performance of the odd and even trays can differ and may be a cause for a hydraulic bottleneck.
In addition to difficulties found in conventional designs of fractionation trays with downcomers in divided wall columns, there are difficulties in conventional columns that do not include a divided wall. Trays require mechanical support that is normally achieved by using the downcomer as the main support structure. The problem is as
column diameters have become larger, downcomer height must be increased in order to meet the criteria for tray deflection. This in turn causes the design tray spacing to increase to accommodate the deeper downcomer. The tray spacing is therefore determined or limited by mechanical constraints and not by process/hydraulic constraints. A significant advantage of the trays is the relatively low tray spacings of about 12 in (30.48 cm) that can be achieved as compared to competitor technologies. At large diameters this advantage begins to erode as the tray spacings need to be increased in order to meet mechanical design criteria. A low tray spacing allows a significant number of MD/ECMD trays to be contained in a single shell or possibly to reduce the column height to meet customer restrictions giving a technical advantage in the marketplace. While I-beams could be used for very large diameter columns greater than 9.7 m (>32 ft.), a relatively high tray spacing is still required. The issue is that in prior art configurations, the I-beams are perpendicular to the downcomers that it is supporting and parallel to the downcomers to the tray below. The beam is therefore directly above the active tray deck below and has the potential to interfere with vapor flow through that particular deck segment that is below it. Another issue is if the beam is wide in being greater than 15 cm (6 in.) there is difficulty getting liquid under the I-beam to feed the active tray deck below. These two concerns can potentially cause maldistribution and poor column performance. In order to reasonably prevent these issues, the I-beam height is limited so it can only extend slightly below the downcomer bottom, therefore limiting its strength.